Pages

Sunday, January 22, 2012

Rails

I started working on getting the rails formed for the LiteBoard kit. I'm using 10mm ABS plastic strips to try to avoid waiting for the pour rails to cure.

I had the local plastics shop cut the ABS off cut sheet I had into 10mm strips so they are all nice and uniform. The side rails are no problem but I wasn't really sure the best way to form the rails around the tip.

ABS is a thermoplastic so heating it up makes it go very pliable but at what temperature is it workable and at what temperature does it burn. Rather than google (cause I dont' have a thermometer to do anything precisely) I experimented on a number of ABS strip offcuts. The approaches I tried were

i) Use a propane torch to heat the plastic through fibreglass matting to minimise the direct contact with the flames
ii) place the rail material inside a narrow metal pipe (and on a bed of fibreglass to keep it from direct contact with the metal to avoid burning it
iii) Use a multiple quick passes of the flame directly on the plastic

Pipe Oven.

On balance (iii) turned out to be the best approach. I simply held the rail by the short end, passed the torch under it back and forwards quickly at a good distance so it was the hot gases and not the flame warming it up and as soon as the plastic started to bend under its own weight it was in the right state to be formed.

ii) Has promise and will probably be the best approach if I can get a temperature probe to monitor the heat. Alternative I could just look up the require temperature and see if our kitchen oven an provide the right amount.

UPDATE: The magic number seems to be 234 degrees f or 112 degrees C. Should have just backed them in the oven!

In the various attempts I scorched the plastic. It didn't burn it but there was brown discoloration on the outside. When it cooled I tried breaking it and the scorched plastic was definitely more brittle - it broken rather than deformed elastically before separating. When I examined inside there was clear small bubbles in the plastic which no doubt would account for some of the reduced strength.

Once I had worked out the approach to softening the plastic I used the board template and the plastic mould pieces to press the rails into shape.

Used the core template and the mould pieces to force the rails into shape. Word like a charm

So, the final piece of the puzzle left to solve is how to be secure the mould piece onto the rocker table surface. One approach would be to just to bolt it in place but I'm reluctant to drill holes in the table surface because it will mean drilling a new set of hole for each mould and template.

I have been wonder if, once its in the vac bag and clamped then this might be enough to hold them in place ( the idea will be to assemble and vac bag the the lot including the rocker table when flat and then bend it in the jigs. So everything will be clamped before bending on the jigs. I' wondering then if the clamping pressure from the vacuum will be enough to hold the mould pieces in place. I think it will come down to how much tension the bag can hold without stretching. I don't think that proper vac film will be very good at this cause its design to stretch over the surface and not bridge. So, I think that using thick PVC film like table protectors might be the way to go.